Oxidation state of Cu in silicate melts at upper mantle conditions

Beyond its economic value, copper (Cu) serves as a valuable tracer of deep magmatic processes due to its close relationship with magmatic sulfide evolution and sensitivity to oxygen fugacity (fO2). However, determining Cu’s oxidation state (+ 1 or + 2) in silicate melts, crucial for interpreting its behavior and reconstructing fO2 in the Earth’s interior, has long been a challenge. This study utilizes X-ray Absorption Near Edge Structure spectroscopy to investigate the Cu oxidation state in hydrous mafic silicate melts equilibrated under diverse fO2 (− 1.8 to 3.1 log units relative to the Fayalite–Magnetite–Quartz buffer), temperature (1150–1300 °C), and pressure (1.0–2.5 GPa) conditions. Our results reveal that Cu predominantly exists as Cu+ across all fO2 conditions, with a minor Cu2+ component. This dominance of Cu+ persists even in relatively oxidized melts, highlighting its limited sensitivity to fO2 under upper mantle conditions. This significantly constrains the utility of Cu as an oxybarometer in hydrous silicate melts in the deep Earth. However, our findings suggest that Cu isotopes primarily reflect the interplay of sulfide segregation/accumulation during magmatic differentiation, shedding light on these fundamental processes in Earth’s interior.


Cu contents in melts and fO 2 estimation
Utilizing Pt 95 Cu 05 alloy capsules, the measured Cu contents in the silicate melts represent their apparent solubilities.Previous studies have established that the solubility of Cu in mafic melts rises with temperature and fO 2 , with minimal pressure dependence at upper mantle conditions 7,10,12,13,21,22 .Notably, Cu solubility exhibits minimal variation among basaltic, andesitic, and dacitic melts at comparable temperatures and fO 2 21 .In this study, Cu contents in silicate melts were measured using LA-ICP-MS and EPMA, ranging from 281 to 6320 ppm (Tables 1 and 2).
Accurate fO 2 values were determinable for runs Cu-8, MORB-L2, and Cu-50, where fO 2 buffer control was successfully established.Methods outlined by Xu et al. 23 and Burnham 24 estimated fO 2 in these runs, yielding a range from ΔFMQ − 1.8 to ΔFMQ + 3.1.Subsequently, the fO 2 conditions in unbuffered runs were estimated based on their respective melt Cu contents, as observed variations in Cu content primarily arise from differences in temperature and fO 2 .The higher Cu contents observed in the 1150 °C runs (Cu-6, 8, 45) compared to those in the 1230-1300 °C runs provide evidence of their elevated fO 2 conditions, supporting the fO 2 estimations obtained for the successfully buffered runs (Table 1).This trend is further validated by comparing the Cu contents across Table 1.Experimental conditions and run products.Cu concentrations in melt, except for runs Cu-8 and Cu-45, have been reported in Liu et al. 7 .fO 2 buffers used were FMQ (fayalite-magnetite-quartz), NNO (Ni-NiO), and MnMnO (MnO-Mn 3 O 4 ).In run Cu-52, the FMQ buffer failed due to the absence of fayalite in the buffer assemblage after experiment.For runs Cu-8, MORB-L2, and Cu-50, fO 2 was calculated based on melt water activity following methods in Xu et al. 23 and Burnham 24 .An estimated relative fO 2 sequence for the unbuffered runs, based on Cu solubility, is provided: Cu-8 > Cu-45 > Cu-6 > Cu-52 > Cu-47 > MORB-L2 > Cu-50 (detailed in the text).The modal abundances of these products were determined through mass balance calculations, as detailed in Liu et al. 7 Gl glass, Ol olivine, Opx orthopyroxene, Cpx clinopyroxene, Spl spinel.www.nature.com/scientificreports/these runs: 6320 ± 320 ppm (ΔFMQ + 3.1) in Cu-8, 420 ± 9 ppm (ΔFMQ − 0.6) in MORB-L2, and 281 ± 8 ppm (ΔFMQ − 1.8) in Cu-50.Therefore, the melt Cu content can serve as a proxy indicator of fO 2 conditions in the unbuffered runs.

Capsule influence minimized on Cu K-edge XANES
This study utilized Pt 95 Cu 05 alloy capsules in piston cylinder experiments to serve as Cu source and prevent Cu loss 7 .However, their significantly higher Cu content (~ 5 wt%) compared to the silicate melts (200-6000 ppm) posed a potential challenge: misleading Cu K-edge XANES spectra due to unwanted contributions from the capsule walls.To address this concern, we implemented a focused beam strategy using a slit collimation system.This strategy confined the synchrotron X-ray beam to a diameter less than 1 mm during non-focusing mode, selectively probing the central region of the glass sample and minimizing the influence of Cu from the surrounding capsule wall.
The effectiveness of this approach is evident in both the minimal Pt fluorescence intensity and the dominant Cu fluorescence intensity observed in the X-ray spectrum (Fig. 1).While the Pt:Cu ratio in the capsule is 19:1, the Cu fluorescence intensity (200-300 counts) far exceeds the expected contribution from the capsule material.This confirms that the focused beam successfully targeted the glass sample, minimizing contributions from the Cu-rich capsule wall.Furthermore, we obtained the XANES spectra of both the capsule wall and quenched silicate glasses in Cu-47 for comparison (Fig. 2).The results reveal distinct spectra for the capsule and glasses, indicating that the influence of the capsule wall on XANES can be readily discerned.Consequently, the prominent Cu peak clearly originates from the glass sample itself, providing reliable data for investigating its Cu valence states and coordination.

Assessment of beam damage
Beam damage in XANES can alter oxidation states through thermal effects and photo-induced oxidation/ reduction.Previous studies observed oxidation of Cu in most samples and reduction of highly oxidized ones (ΔFMQ > 7.4) due to this phenomenon 20 .To evaluate potential beam damage in this study, we employed timedependent monitoring and spectral comparisons at the XANES beamline.We acquired XANES spectra at the same glass location in run Cu-47 three times to monitor for potential changes during analysis.The spectra retained the same overall features throughout, although minor fluctuations, particularly in the pre-edge peak www.nature.com/scientificreports/intensity, were observed (Fig. 2).Comparing spectra collected at different times and locations on the same sample further supported these observations, suggesting minimal impact of beam damage on Cu oxidation state in the silicate melts.Notably, we observed no significant oxidation or reduction attributable to beam damage in our samples.This minimal beam damage effect confirms the reliability of our XANES results in accurately reflecting the Cu oxidation state within the silicate melts.This supports the conclusions drawn from our analysis regarding Cu behavior at upper mantle conditions.

Oxidation state of Cu in silicate melts
Figure 3 exhibits the normalized Cu K-edge XANES spectra of reference standards and quenched glass samples, revealing the dominance of Cu + in these hydrous basaltic melts.Key to this determination is the first peak of the derivative spectrum: 8979.4 eV for Cu 0 , 8980.9 eV for Cu + 2 O, and 8984.0 eV for Cu 2+ O (Fig. 3b).Notably, Fig. 3a shows a characteristic pre-edge peak centered around 8983.4 eV in all melt glasses, confirming Cu + presence due to the 1s → 4p electron excitation 11,20,25 .This peak weakens with increasing coordination number, reflecting orbital interactions with surrounding ligands 20 .While Cu + in silicate melts and Cu + in the Cu 2 O standard share the same oxidation state, their specific XANES features differ due to distinct chemical environments and coordination geometries.Conversely, Cu 2+ exhibits a weak peak around 8979 eV due to the dipole-forbidden 1s → 3d transition, absent in Cu + due to filled 3d orbitals 20,25 .Remarkably, even across a wide range of fO 2 conditions (ΔFMQ − 1.8 to ΔFMQ + 3.1), no systematic correlations between Cu oxidation state and fO 2 were observed in our study.Figure 3 clearly illustrates this lack of dependence, and the unexpectedly weak intensity in run Cu-52 (with conditions similar to the higher-intensity run Cu-47 but under a much more reduced condition) further  reinforces this finding.This finding deviates from Miller et al. 20 , who observed increasing pre-edge intensity with decreasing fO 2 .
While Miller et al. 20  The peak at 8978.8 eV, associated with Cu 0 , only appears in the derivative spectra of the alloy capsule in run Cu-47 (Fig. 2b), along with the standard (Fig. 3).These results collectively indicate Cu + as the dominant valence state in the silicate melts, accompanied by a minor Cu 2+ component.This aligns with the estimation using the empirical equation from Miller et al. 20 , yielding Cu + /ΣCu values of 0.903 in Cu-8 (ΔFMQ + 3.1), 0.983 in MORB-L2 (ΔFMQ − 0.6) and 0.993 in Cu-50 (ΔFMQ − 1.8).
As shown in Fig. 4, temperature, pressure, and melt composition appear to exert minimal influence on Cu's oxidation state within our experimental range, echoing recent findings that Cu + /ΣCu weakly depends on composition, with a slight preference for Cu 2+ in more mafic melts 20 .They also found that increasing temperature stabilizes Cu + , while increasing pressure had little effect on Cu + /ΣCu in mafic melts but preferentially stabilized Cu 2+ O in granite melts.Importantly, arc basalts that form at subduction zones are predominantly produced through fluid-induced melting of the mantle wedge.The quenched glasses in this study, which mimic natural hydrous basaltic magmas under sub-arc mantle conditions, provide a more direct relevance to the interpretation of natural samples.By emphasizing the dominance of Cu + in hydrous arc basalts under these conditions, our work not only corroborates established findings but also contributes valuable insights into the behavior of Cu in such geological settings.

Geological implications
Our XANES analysis, in agreement with recent studies 20 , reveals Cu + as the dominant oxidation state in hydrous mafic melts under upper mantle conditions, exceeding 90% in most cases even under relatively oxidized conditions.This prevailing dominance of Cu + across a broad spectrum of fO 2 values significantly diminishes the sensitivity of the Cu + /Cu 2+ ratio as an indicator for terrestrial fO 2 .As a result, the effectiveness of the Cu + /ΣCu ratio as an oxybarometer to monitor the genesis and evolution of hydrous basaltic magmas within subduction zones is compromised, as the Cu + /ΣCu ratio only fluctuates from approximately 90% to 98%.However, it unlocks new avenues for understanding magmatic processes through Cu isotopes.Previous studies have demonstrated that Cu isotope fractionation can occur during: (a) sulfide-silicate melt partitioning, where Cu preferentially concentrates in sulfide, leading to isotopically heavier equilibrium silicate melts 1,4 ; (b) fluid-silicate melt partitioning, where exsolved chlorine-bearing fluids have higher δ 65 Cu than the residual magmas 19 ; and (c) electron-exchange-driven fractionation, where changes in Cu oxidation state, such as Cu + to Cu 2+ , can result in isotopic fractionation 26 .Crucially, our finding of Cu + 's dominance under diverse fO 2 conditions suggests that Cu oxidation state changes are unlikely to be the primary driver of Cu isotope variations in natural samples.Instead, the observed isotopic signatures likely reflect the interplay of sulfide segregation/accumulation or fluid exsolution during magmatic differentiation.For example, given the affinity of Cu + for sulfide phases, its geochemical behavior is closely linked to the evolution of sulfur during magma generation, ascent, and cooling 1 .Under upper mantle conditions, Cu is primarily hosted in Cu-poor monosulfide solid solutions or sulfide melts, with minimal partitioning into other phases 1,2,7 .This accounts for the limited isotopic fractionation observed during partial melting of the mantle, even in the presence of sulfide residues 1 .However, as magmas evolve and cool at lower crustal levels, the formation of Cu-rich sulfides leads to substantial isotopic fractionation, with isotopically heavier Cu preferentially incorporated into these sulfide phases.Additionally, chlorine-rich fluid saturation at depth can also lead to lighter δ 65 Cu in residual phases.By decoupling redox variations from isotopic signatures, we can gain clearer insights into the evolution of sulfur within magmatic systems, which is not solely related to redox changes 23 .This newfound understanding can illuminate critical aspects of magmatic differentiation, including the formation of ore deposits and the behavior of volatile elements.

Starting materials
This study examined the valence state of Cu in different silicate melts using three mafic compositions-komatiite, mid-ocean ridge basalt (MORB), and Fe-free Di 70 An 30 (diopside-anorthite) (Table S1).Reagent-grade oxides (SiO 2 , Al 2 O 3 , Fe 2 O 3 , MgO, TiO 2 , MnO, NiO, P 2 O 5 and Cr 2 O 3 ) and carbonates (CaCO 3 , Na 2 CO 3 , and K 2 CO 3 ) were mixed, ground under acetone, and successively sintered at 1000 °C for ten hours and fused at 1500 °C for two hours in platinum crucibles to remove CO 2 and ensure chemical homogeneity.Subsequently, the crucibles were rapidly cooled by immersion in purified water, and the quenched glass was finely ground to yield homogeneous glass powder.Two rounds of this fusion-grinding process were employed to achieve complete decarbonation and enhance melt consistency.For more details, refer to Liu et al. 7 , where a portion of the Cu solubility results were previously reported.

Sample capsules and fO 2 control
We employed the same experimental setup as described in Liu et al. 7 , utilizing Pt 95 Cu 05 alloy capsules in two sizes: a smaller sample container (ID: 2.7 mm, OD: 3.0 mm) and a larger capsule for sample and oxygen fugacity solid buffer (ID: 4.7 mm, OD: 5.0 mm).Each capsule, except for Cu-50 with 1 wt% H 2 O, received approximately 15 mg of initial silicate powder and 5-6 wt% H 2 O before welding.The added H 2 O facilitates equilibration and promotes the formation of crystal-free basaltic glasses during the experiments.Notably, in Cu-50, the sample capsule was loaded into a graphite capsule within a larger alloy capsule, with one end welded and the other crimped for communication with the graphite.Following welding, all capsules underwent drying to confirm no leaks.The alloy capsule also serves as the source of Cu.With a high mass ratio of the capsule (12-16 times the silicate charge), the alloy capsule acts as a buffer, maintaining a constant Cu activity or concentration in each phase of the silicate charge under specific P-T-fO 2 conditions.This approach guarantees Cu homogeneity within the melts and circumvents the disequilibrium issues commonly associated with the "Cu-loss problem".Oxygen fugacity of primitive arc basalts spans a wide range, typically extending from ΔFMQ-2.0 to ΔFMQ + 3.5, with most clustering between FMQ and ΔFMQ + 2.0 3,27 .To replicate this diversity in the study, a conventional double-capsule technique utilizing Fayalite-Magnetite-Quartz (FMQ), Ni-NiO (NNO), and MnO-Mn 3 O 4 (MnMnO) buffers was employed to control the experimental fO 2 , following the methodology outlined in Liu et al. 7 .In this configuration, a sealed sample capsule containing the silicate powder is welded into an outer alloy capsule.The space between the capsules is then filled with the specific buffer material and H 2 O. It's important to note that the actual fO 2 should be slightly lower than the imposed buffer due to the H 2 O activity in the silicate melt being less than unity 23 .On the other hand, three unbuffered experiments were conducted using a single sample capsule placed inside an MgO tube with crushable spacers and MgO powder filling the remaining space.In these runs, the fO 2 is believed to be primarily imposed by the starting material and the cell assembly.

Piston cylinder experiments
Primitive arc basalts form under a range of temperature and pressure conditions, typically spanning 1150-1350 °C and 0.8-2.1 GPa, respectively 27 .To simulate these conditions and investigate Cu behavior in arc basalts, our experiments employed high pressures (1.0-2.5 GPa) and temperatures (1150-1300 °C) with durations of 27-74 h using end-loaded piston-cylinder apparatuses (details in Table 1).Six experiments at 1.0 GPa were conducted in a 3/4 inch pressure vessel at the Guangzhou Institute of Geochemistry, while one experiment at 2.5 GPa (run MORB-L2) used a 1/2 inch vessel at the Bayerisches Geoinstitut.Each assembly consisted of an outer NaCl/ talc + Pyrex sleeve and a tapered graphite heater (NaCl used in Guangzhou, talc in Bayreuth).The sample capsule, housed in a pyrophyllite or MgO sleeve, was positioned at the center of the heater within Al 2 O 3 spacers.The hot piston-in method applied pressure, automatically regulated throughout the experiment.Pressure values were further corrected for friction based on the specific assembly used (3% for NaCl + Pyrex, 18% for talc + Pyrex).Temperature control was achieved using Pt/Pt 90 Rh 10 thermocouples and a Eurotherm controller, maintaining a deviation of ± 2 °C from the nominal temperature during the experiment.An estimated uncertainty of ± 15 °C was considered due to the temperature gradient within the capsule.Quenching was achieved by rapid cooling upon switching off the power.Each capsule was then carefully extracted from the assembly, mounted in epoxy, and polished for subsequent optical and chemical analyses.

EPMA and LA-ICP-MS
Major element and CuO concentrations in both minerals and quenched glasses were determined using electron probe microanalysis (EPMA) on two JEOL JXA microprobes: the JXA-8100 at the Guangzhou Institute of Geochemistry and the JXA-8200 at the Bayerisches Geoinstitut.The analysis employed a focused beam for minerals and a 20 µm beam for quenched glasses.The accelerating voltage was set at 15 kV for all elements in minerals and 20 nA for Cu, while other elements in quenched glasses were analyzed at 10 nA.Counting times were set at 20 s for all elements, except Na, K, and Cu, where 10 s on the peaks for Na and K, and 40 s on the peak for Cu were used.The detection limit for Cu was approximately 350 ppm.Wavelength-dispersive spectrometry (WDS) was employed for analysis, and the PAP matrix correction was applied to raw data.Standards used included andradite (Si), MnTiO 3 (Ti), spinel (Al), metal Fe (Fe), MnTiO 3 (Mn), forsterite (Mg), wollastonite (Ca), albite (Na), orthoclase (K), gallium phosphite (P), metal Ni (Ni), metal Cr (Cr), and metal Cu (Cu).Excellent agreement between measurements (major elements and CuO) obtained from both instruments in Guangzhou and Bayreuth was achieved 7 .
Cu concentrations in quenched glasses were determined using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at either the Bayerisches Geoinstitut (run MORB-L2) or the Guangzhou Institute of Geochemistry (run Cu-6, -45, -52, -47, and Cu-50 www.nature.com/scientificreports/systems: a Geolas M with an Elan DRC-e at Bayreuth and a Resonetic with an Agilent 7500a at Guangzhou.Laser operation parameters ranged from 5 to 10 Hz repetition rate, 80 mJ energy, and 20-80 µm spot size (typically 30 µm).The sample chamber was flushed with He at 0.4 L/min, with 5 ml/min of H 2 added to the carrier gas to enhance Cu sensitivity.We achieved a detection limit of 0.1 ppm for Cu.NIST SRM 610 glass served as the external standard, while Si content determined by electron microprobe analysis provided the internal standard.
The measured Cu concentration in the SRM 610 standard exhibited a reproducibility (1σ) of < 10%.For further details on the LA-ICP-MS methodology, please refer to Liu et al. 7 .

XANES
The oxidation state of Cu in quenched glasses from piston cylinder experiments was investigated using Cu K-edge XANES (X-ray Absorption Near Edge Structure) spectra at the Shanghai Synchrotron Radiation Facility (BL14W1 beamline station).Measurements were conducted under well-defined beam conditions, with a stored ring electron energy of 3.For accurate energy calibration, the first derivative peak in the Cu foil XANES spectrum (transmission mode) was aligned to 8978.9 eV before sample measurements.The scanning range spanned 180 eV pre-edge to 200 eV post-edge, optimizing extended edge analysis for Cu coordination, particularly oxidation state.CuO 2 and CuO standards were finely powdered and tape-mounted for transmission measurement.Spectra were recorded from 8778.9 to 9175.2 eV with a step size of 5 eV up to 8959 eV, 0.3 eV between 8959 and 9030 eV, 1 eV between 9030 and 9079 eV, and 2 eV above 9079 eV.The count time was 2 s per point.Signals from the 32-element detector were averaged, and resulting spectra were normalized using ATHENA.A total of seven samples underwent analysis through Cu K-edge XANES spectra.The first set of samples, analyzed using the non-focusing mode, comprised runs Cu-47, Cu-8, MORB-L2, Cu-6, and Cu-52.In contrast, the second set, which employed the focusing mode, included samples Cu-45, Cu-50, and Cu-52.To ensure the reliability of the results, each sample underwent multiple measurements, minimizing potential interference in the analysis process.

Figure 1 .
Figure 1.Fluorescence spectrum of sample Cu-6, indicating limited influence from the Pt 95 Cu 05 alloy capsule during the XANES measurement.

Figure 2 .
Figure 2. (a) Cu K-edge XANES spectra and (b) corresponding derivative spectra for the Pt 95 Cu 05 alloy capsule and the enclosed quenched silicate glass from run Cu-47.This comparison allows for examination of the influence of the capsule material on the Cu signal in the glass and ensures the integrity of the data acquired from the sample.The XANES spectra were acquired three times on the same glass location to evaluate potential beam damage during the analysis.
observed features indicating Cu 2+ in oxidized samples (ΔFMQ > 5.1), our spectra lack the prominent 1s → 3d transition peak at 8979 eV and the shoulder attributed to Cu 2+ O.However, faint echoes of these features appear in the most oxidized runs (Cu-8, 45, 6, and 52) as weak flat peaks near ~ 8991 eV in the derivative spectra, potentially corresponding to the α and β peaks observed by Miller et al. 20 at ~ 8984.1 eV and ~ 8991.4 eV, respectively.These peaks, attributed to Cu 2+ O in an octahedral coordination environment, are significantly less pronounced in our samples compared to the CuO standard, suggesting a very minor contribution of Cu 2+ O alongside the overwhelming dominance of Cu + .

Figure 3 .
Figure 3.Comparison of Cu K-edge XANES and derivative spectra for Cu reference standards and quenched hydrous basaltic glasses.The pre-edge peak in the derivative spectra highlights Cu + as the dominant valence state in the glasses.Spectra are organized by decreasing fO 2 (top to bottom).

5
GeV and a beam current ranging from 150 to 210 mA.The ionization chamber was filled with N 2 to reduce attenuation and scatter.Two sets of analyses with fluorescence mode were conducted: (a) non-focusing mode: utilized a double-crystal Si (311) monochromator with a spot size of 800 × 400 µm and a 32-element high-purity Ge solid-state detector for X-ray absorption spectra collection; and (b) focusing mode: employed a Si (111) monochromator with a spot size of 200 × 200 µm and a Lytle fluorescence ionization chamber for signal collection.Both approaches effectively measured the Cu K-edge absorption edge for samples with Cu content exceeding 100 ppm.

Table 2 .
20mposition (wt%) and Cu concentrations (ppm) in quenched glasses.Glass compositions, except for runs Cu-8 and Cu-45, are reported in Liu et al.7.H 2 O contents were estimated by difference, calculated as 100% minus the sum of all other oxide components.Optical basicity values were calculated as described in Miller et al.20.n: number of analyses.Numbers in brackets represent ± 1σ standard deviation.n.a.= not analyzed.